This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-216988, filed Jul. 17, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a transmission system which performs digital transmission of video data and audio data, and more particularly to improvement in an audio clock restoring apparatus and an audio clock restoring method which restore a sampling clock of audio data on its reception side.
2. Description of the Related Art
As is well known, in recent years, there becomes widely used a digital transmission system which transmits video data, audio data, a sampling clock of the video data and information indicative of the relationship between the sampling clock of the video data and the sampling clock of the audio data but does not transmit the sampling clock of the audio data itself.
In this digital transmission system, the sampling clock of the audio data must be restored on the reception side based on the sampling clock of the video data and information indicative of the relationship between the sampling clock of the video data and the sampling clock of the audio data.
Meanwhile, for restoring the sampling clock of the audio data as described above, the most important point lies in that a frequency or a phase of the sampling clock of the audio data to be restored on the reception side must coincide with a frequency or a phase of the sampling clock of the audio data used on the transmission side.
In view of the above-described problem, it is an object of the present invention to provide an audio clock restoring apparatus and an audio clock restoring method which can match a frequency or a phase of an audio clock to be restored on the reception side with a frequency or a phase of an audio clock on the transmission side.
According to one aspect of the present invention, there is provided an audio clock restoring apparatus to which video data, audio data, a video clock and relevant information indicative of a phase difference between the video clock and an audio clock are inputted and which restores a restored audio clock based on the video clock and the relevant information, comprising:
a reference phase signal generation portion configured to generate a reference phase signal from the video clock and the relevant information;
a feedback phase signal generation portion configured to generate a feedback phase signal from a restored audio clock;
a first phase difference detection portion configured to compare the reference phase signal with the feedback phase signal in phase and to output a first error signal corresponding to a phase difference between the reference phase signal and the feedback phase signal;
a first oscillation portion whose oscillation frequency is based on the first error signal configured to output a first clock;
a second phase difference detection portion configured to compare the first clock with the restored audio clock in phase and to output a second error signal corresponding to a phase difference between the first clock and the restored audio clock; and
a second oscillation portion whose oscillation frequency is based on the second error signal configured to output the restored audio clock.
According to another aspect of the present invention, there is provided an audio clock restoring method by which video data, audio data, a video clock and relevant information indicative of a phase difference between the video clock and the audio clock are inputted and which restores a restored audio clock based on the video clock and the relevant information, comprising:
generating a reference phase signal from the video clock and the relevant information;
generating a feedback phase signal from a restored audio clock;
comparing the reference phase signal with the feedback phase signal in phase and obtaining a first error signal corresponding to a phase difference between the reference phase signal and the feedback phase signal;
controlling an oscillation frequency of a first oscillation portion based on the first error signal;
comparing a first output clock from the first oscillation portion with the restored audio clock in phase and obtaining a second error signal corresponding to a phase difference between the first output clock and the restored audio clock; and
controlling an oscillation frequency of a second oscillation portion based on the second error signal.
According to the structure and the method mentioned above, the oscillation frequency of the first oscillation portion is controlled based on a result of comparing the reference phase signal with the restored audio clock in phase, and the oscillation frequency of the second oscillation portion is controlled based on a result of comparing the output clock of the first oscillation portion with the restored audio clock in phase.
That is, since the restored audio clock is a target of comparison in two error detections, the audio clock to be restored can be accurately synchronized with the audio clock on the transmission side as compared with the structure in which two PLLs are simply connected to each other in the cascade manner.